Well, for game performance..
Like Razear said, defrag, remove stuffs, all sorts of that. But what I do is I clean my registry and defrag often, Disable anti-virus or any programs running in the background. Also there must be an option on your video card to adjust for quality or for performance.

And for the game itself, (If you dont mind the slight lowering of quality for your games) I would lower the resolution, lower the texture quality, and well disable any of the graphical additions, like hi-def shadows, fogs, smooth edges etc.

Try doing upgrades to your computer.
Get another bigger hdd. 500gb or 1TB
Get another 2x1gb ram.
Replace the old graphics card with a new 1GB GPU.
And maybe cooling might be a problem, so cleaning your case, and adding more fans will improve the cooling of your components, thus making it work more efficiently.

Your bottleneck as far as gaming is concerned is your graphics card. If you can upgrade it to something a bit newer you would see a lot of improvement. The Pentium 4 may not be all too new but it is at 3ghz. In the game settings, if you set everything down to the lowest but keep it at your monitors native resolution, you won't notice the quality as much.